Information transfer devices read and/or write information from/to a storage medium. The transfer devices include tape drive devices in which the medium comprises a length of tape wound on at least one reel, which is drawn past a data transfer head. The data transfer head includes at least one data transfer element, i.e., transducer, which in use transfers information onto the medium or reads information from the medium. In some instances, a single data transfer head performs both read and write functions. With a magnetic tape drive, information is stored magnetically by applying a write current to the element (i.e. electric-magnetic transducer) as tape is drawn past the element. Information is read by from the tape by measuring an output current from the element as the tape is drawn past the head. A disk drive is similar in principle except that a rotating disk carrying the recorded information rotates past the information transfer head, i.e., transducer.
When reading information from a storage medium a data transfer element produces a signal that depends on the information stored on the medium being read. The element typically produces an analogue output signal which is passed through an amplifier to a read channel circuit which processes the analogue signal. In many devices the read head comprises multiple data transfer elements that read information from multiple tracks of a storage medium at the same time. In the case of a magneto-resistive (MR) data transfer head, each data transfer element can be represented by the electrical equivalent circuit shown in FIG. 1 of the accompanying drawings as a simple resistive element which is designated by the reference numeral 10. When a magnetic field, or flux, is applied to the head element the resistance of the element 10 varies. If a constant bias current IBC is caused to flow through the resistive element a voltage Vout is produced across the resistive element 10 which can be amplified.
Specifically, as shown in FIG. 2 of the accompanying drawings, the magnitude of the voltage across the resistive element 10 is related by a non-linear transfer function 24 to the sinusoidal flux 21 applied to the element. In FIG. 2 the horizontal axis 23 represents the flux applied to the element 10 and the vertical axis 22 represents the output voltage Vout. The Figure shows that the output voltage waveform 24 varies between Vmax and Vmin as the flux varies between Xmax and Xmin. This allows data to be encoded on a magnetic medium (tape or disk or the like) by changing the flux of the medium to be read by the data transfer element. Because several data transfer elements are provided, each producing its own output voltage, several tracks of data can be provided in parallel on the tape or disk which can be read simultaneously, increasing the rate of data transfer for a given tape or disk speed.
Magneto Resistive (MR) information transfer elements suffer from a non-linear distortion in their output voltage/flux characteristic 20. This is because the transfer function of MR data transfer elements is non-linear and depends, inter alia, upon the intrinsic properties of the element and on the properties of the medium. One such property that affects the linearity is the coercivity of the magnetic recording medium. This distortion is often referred to as amplitude asymmetry.
The transfer function 20 shown in FIG. 2 is clearly non-linear and, despite using a bias current that places the output voltage in the most linear region of the transfer function, the output voltage waveform 24 responsive to the sinusoidal flux variation 21 is a distorted sinusoid. This non-linearity introduces amplitude asymmetry to the transfer signal produced by the element (i.e. the positive and negative cycles are not the same). For an information transfer head having multiple elements, the distortion—and hence the amplitude asymmetry—for each element is independent as each element has slightly different properties. This can significantly degrade the performance of the data transfer device.
It can be shown that the transfer function 20 of an MR element can be approximated by the relationship:Vout=Vin+Vin2*K  (1)where Vout is the output voltage measured across the element and Vin is the input signal representing the magnetic flux and K is an example of a compensation value which is usually <0 and a constant for a given combination of magneto-resistive element and coercivity of storage medium.
If the compensation value, K, is chosen appropriately it is possible to reduce the effects of the asymmetry for a given coercivity of storage medium. The appropriate compensation value can be chosen by selecting an initial compensation value and observing the asymmetry exhibited by the element when used to read information from the storage medium. If the output is not symmetrical the compensation value is inappropriate and should be increased or decreased slightly and the test repeated. The test can be continued many times until the most appropriate compensation value is found, i.e. the one that minimises the asymmetry of the output from the element at the given coercivity or brings it within acceptable limits.
For a multi-transfer element head an appropriate value of K must be chosen for each data transfer element since each data transfer element has a different characteristic. The result of the tests is a data transfer head that exhibits little asymmetry for the given coercivity. The tests should be repeated if the coercivity of the medium changes so that a new set of appropriate values of K can be obtained.
Another solution to the amplitude asymmetry problem is the use of a non-linear analogue to digital converter which converts the output voltage from the head element to a digital signal. The non-linearity of the converter is chosen to be the reciprocal of the non-linearity of the data transfer element so the effects of non-linearity can be removed. As with the use of calibration values K, this works well with a data transfer device having a fixed storage medium with a given coercivity, but it is of limited use with a device that uses a removable storage medium. Whenever a new storage medium is presented to the device, which may have a different coercivity, the distortion will also change, as the non-linearity of the amplifier may no longer be appropriate.